Lord Winston of Hammersmith, one of Britain's leading medical experts and a Labour peer, has said the soaring costs of the NHS mean that in the future patients will inevitably have to make a contribution towards the cost of any treatment they receive.

He attacked the Government and the Conservatives for being afraid to launch a debate on exactly how the health service should be funded in the future.

Lord Winston's remarks came the day after the Conservative leader, David Cameron, said he would never advocate abandoning the principle of a taxpayer-funded NHS that was free at the point of delivery.

Mr Cameron has also scrapped his party's much-maligned "patient passport" policy that would have allowed people to claim NHS money to fund private operations. He launched a policy group on public service improvement yesterday, which will look at ways of freeing health and education professionals from excessive Whitehall interference.

Senior doctors, including Lord Winston, have been frequent critics of the targets and regulation imposed on them since Labour came to power.

The Government has insisted that its use of the private sector to provide speedier treatment and more patient choice will not threaten the founding principles of the health service.

However, Lord Winston, who pioneered fertility treatment in the UK, said the cost of the NHS was rising too steeply and expensive new treatments were coming on stream too quickly for governments to cope with.

Lord Winston said: "One of the biggest issues that we are facing is that no government, including this one, has ever really acknowledged where the costs of the NHS are finally likely to be.

"The rising cost is absolutely massive and there is no sign of that regressing. I am not convinced that any political party is realistic about the costs of the NHS."

The NHS currently costs almost £70bn a year to run and the annual budget is expected to rise to £94.3bn by 2008. Lord Winston said that despite increases in funding, an "unacceptable" postcode lottery still existed, where patients in some parts of the country were denied treatment that was freely available to people in other regions.

The Labour peer said: "I know I will get a hammering from some of my colleagues on the Labour benches for saying this, but we need a real debate about how we are actually going to pay for the NHS.

"It doesn't seem to be beyond the realms of probability that patients will have to contribute to a little part of the cost of their treatment. At the moment, it seems to be completely politically incorrect to even suggest that."

The shadow Health Secretary, Andrew Lansley, said: "The Conservatives are committed to a continuation of a free NHS. If given proper freedoms and proper incentives, the NHS could achieve much greater levels of efficiency, which would give us much better value for money."

Lord Winston also attacked the lack of funding for fertility treatment on the NHS, saying that childlessness was a "disease" that was not given the priority it deserved.

Patients are supposed to be given at least one free cycle of fertility treatment on the NHS, but many primary care trusts are imposing restrictions on exactly who is eligible.